As a magnetic sensor for detecting a magnetism or a magnetic field with a high sensitivity, a magneto-impedance element which utilizes magneto-impedance effect is known. In the magneto-impedance element, a constant current of a high frequency is passed through a conductor provided adjacent to the magnetic poles of a soft magnetic body. When the magneto-impedance element is placed in a magnetic field, the impedance of the conductor is changed by the magnetic field. A magnetism is detected on the basis of the change of a high frequency voltage due to this change of the impedance of the conductor.
Part (a) of FIG. 42 is a perspective view of a conventional magneto-impedance element which is reported in the Electronics Information Society Technical Report MR 95-80. Referring to the figure, a detection conductor film 101 formed of a conductive metal thin film is interposed between soft magnetic cores 105 and 106. Each of the magnetic cores 105 and 106 is, as shown by a cross-sectional view of part (b) of FIG. 42, a layered product of a permalloy film 103 and a SiO2 film 104. A high frequency current 109, which is a constant current of a UHF carrier signal, is caused to flow through the detection conductor film 101 from a high frequency generator 107 of UHF band via a resistor 108. A voltage change based on the change of impedance due to the external magnetic field shown by arrow 116 is detected across terminals 110 and 111 of conductive wires which are connected to respective ends of the detection conductor film 101. A carrier signal voltage corresponding to the product of a high frequency current 109 and the impedance across the terminals 110 and 111 of the detection conductor film 101 is generated across the terminals 110 and 111. When an external magnetic field 116 exists, an orientation direction of easy axis of magnetization of the soft magnetic cores 105, 106 oriented in the direction shown by an arrow 102 deviates from the orientation direction due to the external magnetic field 116. As a result, the impedance of the detection conductor film 101 decreases in comparison with the case that no external magnetic field exists. The decrease of the impedance is detected in the form of an amplitude modulation signal of the carrier signal due to the external magnetic field 116. The external magnetic field 116 can be detected by carrying out AM detection to the amplitude modulation signal. The magnetic sensor which utilizes the above-mentioned magneto-impedance effect has a possibility of obtaining a detection output as about ten times high as that of a giant magneto-resistance element which is now in the process of developing at present.
FIG. 43 shows a characteristic curve showing operation of the magneto-impedance element shown in part (a) of FIG. 42, and shows a relation of a magnetic field magnitude DC to a carrier signal level. The frequency of the carrier signal is 1.0 GHz, and the magneto-impedance element is placed in the central portion of Helmholtz coils, and thereto a direct current (DC) magnetic field is applied, and then the magnetic field magnitudes and a carrier signal level are measured. In order to obtain a detection output including a smaller amount of distortion with a high sensitivity, as shown in FIG. 43, it is necessary to give a direct current bias magnetic field of a magnetic field magnitude as shown by a straight line 112.
The impedance of the magneto-impedance element is proportional to the product of the frequency of the carrier signal and a permeability. Therefore, in the case that the frequency is high as mentioned above, a sufficient impedance change is available. However, in the use for the magnetic sensor, there is a case requiring use of a carrier signal of a relatively low frequency. For example, in the case of detecting an external magnetic field of a low level such as geomagnetism, it is required to use a carrier signal of 10 MHz–20 MHz which is far lower than 1 GHz. Accordingly, a magneto-impedance element is demanded whereby a sufficient impedance change is obtainable with a carrier signal of such a relatively low frequency.